1. Field of the Invention
The present invention relates generally to the field of sand cast molding and to improvements in the founding of iron (and other metals) more particularly to improvements in sand molding media employed in forming molds into which molten iron is poured in the production of castings as it relates to the recovering of the molding media in a foundry.
2. Description of the Background
Green sand casting is a well-known process for forming cast metal articles. In this process, a casting mold for making castings, formed from molding media that is primarily sand and bentonite clay for the production of one or multiple castings. Once the casting solidifies in the mold, the mold is broken down and the casting cycle is complete. A portion of the molding media can be recycled for another casting process, however, a substantial portion of the molding media exits the foundry as foundry waste. In the U.S. alone, foundry waste accumulates at a rate of approximately 6 to 10 million cubic yards per year. The large volume of foundry waste coupled with the increasing cost of landfill acreage and transportation is problematic.
Founding is an ancient art in which a cavity is defined in a sand mold and then molten metal poured therein. After the metal cools, the cast article is removed, with the sand mold usually being broken up in the removal process. The usual and basic procedure for forming such sand molds is to compact a sand molding medium around a pattern and then to remove the pattern, leaving a cavity having the configuration of the pattern.
In order for the sand to maintain its molded, cavity-defining configuration, a binding agent that causes the sand particles to cohere is included in the mixture. Clay has long been an accepted and suitable binding agent. Clay is a generic term and encompasses a large group of hydrous alumino-silicate minerals. Individual mineral grains vary in size down to microscopic dimensions. When dampened, clay is tenacious and plastic. When dampened and then dried clay becomes hard, particularly when dried at elevated temperatures. Wet bentonite product performs better under casting conditions.
The present invention is particularly useful in founding where so-called green sand casting is a standard practice. Green sanding casting encompasses a process wherein molten metal is poured into a sand mold while it still retains the moisture that has been added to actuate the cohesive properties of the clay. Sand molding media for iron founding comprise three basic components, namely sand, clay, and finely ground a bituminous coal, commonly known in the trade as “sea coal,” In use, a sand molding medium is moistened with water to provide a medium that is capable of being compacted around a pattern to form a mold cavity. The green sand molds typically comprise by weight, from about 86% to 90% sand and multiple non-sand components, including 8% to 10% bentonite clay, 2% to 4% organic additives, and 2% to 4% moisture. After removal of the pattern, molten iron is poured into the mold cavity while the sand molding medium is still in its dampened or “green” condition. The sea coal on and immediately adjacent the mold cavity surface decomposes under the heat of the molten iron as it is poured into the mold. A product of this decomposition is elemental carbon, in the form of graphite, at the interface between the mold cavity and the poured iron. This elemental graphite serves the primary function of enabling the solidified casting to be released from the mold, free of sand particles. A secondary benefit of the elemental graphite is that it tends to level the surface of the mold cavity, thereby producing a smoother surface on the cast article.
A well-established practice of the trade is for a foundry to purchase a “pre-mix,” which includes a clay component and carbon component. The foundry then mixes the pre-mix with sand from a local source to provide the sand molding media used in operations.
Sand molding media has sufficient cohesive strength of the sand molding medium is most critical in its “green” condition, that is, when it is moistened. After being compacted to define a cavity, the green molding medium preferably has sufficient strength to withstand any forces incident to removal of a pattern, so that the cavity configuration is maintained intact. Next, sand molding media, when in a green stage, preferably has sufficient strength to withstand the forces incident to the mold being moved and repositioned in various fashions in the process preparing it for the pouring of metal into the cavity. Further, the sand molding media preferably has sufficient cohesive strength to withstand the hydraulic forces incident to pouring molten iron into the cavity.
Drying of a green mold occurs extremely rapidly and can occur while the metal is still molten and continues to exert hydraulic forces on the mold structure. The dry strength of the molding medium is therefore critical in assuring that the integrity of the mold will be maintained to the end of obtaining cast articles of the proper configuration.
Another significant, objective characteristic of sand molding media is permeability. A relatively high permeability is preferred in order to prevent damage to the mold when molten iron is poured into the mold cavity. This is to point out that when molten metal is poured into the mold cavity, air is displaced though the mold medium. More importantly, because the sand molding medium is damp, steam can be generated in a rather violent, or explosive, fashion. Such steam is preferably vented through the molding medium with a minimum of gas flow resistance. As such, porous mold structures preferably have a relatively high gas permeability. Strength characteristics and permeabilities are capable of objective determination and acceptable green and dry strengths for sand molding media, as well as permeabilities, are now established.
After an item has been cast, the sand mold is broken up and then accumulated for reuse. The excess molding media, that is, foundry waste which cannot be reused for subsequent casting cycles, is generated at several locations within the foundry. The composition and particle size distribution of foundry waste can vary depending upon the areas of the foundry in which it is collected, but foundry waste can be generally classified in two broad categories, namely, “molding waste” and “bag house dust/dust from mechanical reclamation.” The phrase “molding waste” refers to the excess molding media from broken-down green sand molds and cores, output stream, produced during shakeout. In many green sand foundries, the molding waste typically contains by weight from about 80% to about 90% sand, from about 6% to about 10% bentonite clay and from about 1% to about 4% organic additives. Molding waste includes sand that is coated with bond as well as individual particles of sand, bentonite, and organic additives.
Attempts have been made to reduce the accumulation of molding waste by mechanical reclamation removing the bond from the sand so that the sand is sufficiently clean to be reused in the production of cores. In such processes the sand is recovered, but the bentonite clay, which costs several times more than sand on a weight basis, and the organic additives can be recovered. A disadvantage of mechanical reclamation is that the cost of prime sand is sufficiently low in many geographic areas that the capital investment for sand recovery is economically unfeasible.
A large source of foundry waste stream includes fine particles of sand, bentonite clay, organic additives, and debris collected in the foundry's air evacuation system. This foundry waste is commonly known in foundries as “bag house dust.” Bag house dust contains substantially more bentonite clay than does molding waste. Bag house dust typically comprises from about 40% to about 70% sand, from about 20% to about 50% bentonite clay, and from about 10% to about 30% organic additives. Previous efforts have disclosed hydraulic and mechanical separation processes that reclaim sand, bentonite, and organic components of bag house dust, though the utility of that recovery product is currently unknown. See U.S. Pat. Nos. 6,554,049 and 6,834,706.
Accordingly, there is a long-standing need to reduce the amount of foundry waste exiting a green sand foundry. There also exists a long-standing need for a process to recover sand that has sufficient quality to be used in the foundry to make cores and green sand molds and which can yield quality castings in a subsequent casting process. A process to recover sand, bentonite clay, and organic additives to decrease the amount of prime materials (pre-mix) that enter the foundry as raw material is also needed within the art. Over a period of time it becomes necessary to add fresh amounts of the clay and coal additive. Similarly, it is a common practice to also add fresh sand. This not only maintains a more or less constant ratio of the sand, clay, and coal components, but also compensates for the accumulation of ash that is a byproduct of the decomposition of the sea coal.
The pre-mix discussed above, which includes a clay component and carbon component, has found acceptance in the art because of several advantages. Primarily these advantages are found in the ability to minimize costs by the use of less pre-mix and/or by reducing the total amount of carbonaceous material in the pre-mix. Further, it was demonstrated that the amount of additional, “make-up” pre-mix used in recycling a sand molding medium was reduced.
Another factor to note is that as green sand molding medium is compacted around a pattern (in the normal case) to form a mold cavity. The characteristics of the sand molding medium can have a great impact on the “workability” of the medium and the ability to compact (i.e., densify) the medium and also the ease with which densification can be attained, which is understood as flowability. This factor is relevant to the fact that both the green strength and dry strength of a sand molding medium are directly proportionate to the density of the sand molding medium after it has been compacted to define a mold cavity. There is thus a preference within the art for sand molding media that have a workability characteristic which facilitates obtaining a desired, relatively high, and consistent density of the compacted molding medium. While the workability characteristic is subjective, it is, nonetheless, a recognized standard for sand molding media.